(1) Field of the Invention
The present invention relates to a multi-piece golf ball having a multi-layered structure and to a method for manufacturing the same.
(2) Description of the Related Art
Recently, several golf balls exhibiting both high ball bounce resilience and a soft feel when hit have been proposed. One example of such golf balls is multi-piece golf ball in which the ball is composed of a plurality of layers. Generally, in a multi-layered golf ball, especially in a golf gall that has three or more layers, a highly rigid core is covered with an intermediate layer that has relatively low rigidity, and the outer surface of the intermediate layer is covered with a hard cover. This arrangement aims to attain both high ball bounce resilience and a soft feel when hit by using the rigidity of the core and the softness of the intermediate layer.
However, in such a multi-layered golf ball, when the ball is hit, the intermediate layer is depressed in the portion that received the strike. The ball then rolls while being deformed not only in the layer's thickness direction but also in the direction along the spherical surface, and therefore the striking force tends to be dispersed in the direction along the spherical surface. This poses a problem such that, although a soft feel when hit can be obtained, high ball bounce resilience is difficult to obtain.
In manufacturing a multi-layered golf ball, it is important to accurately form each layer concentrically. Therefore, for example, in a three-layered golf ball that comprises a core, an intermediate layer, and a cover, a manufacturing method explained below has been proposed.
First, a core that will become the center of the ball is molded. Then, a pair of hemispherical, shell-like pieces for forming the intermediate layer are molded in a semi-vulcanized condition. Then, these pieces for forming the intermediate layer are set in the mold, the core that has been molded in advance is inserted therein, and all are subjected to full vulcanization by press molding. Thereby, the core will be arranged in the intermediate layer. Thereafter, a cover is molded onto the outer side of the intermediate layer by injection molding or the like.
However, in this manufacturing method, the core is held by the pieces for intermediate layer that are in a semi-vulcanized condition, and therefore the core may be displaced from the center of the ball. This makes it difficult to concentrically align the core and the intermediate layer in an accurate manner.
Alternatively, the following manufacturing method has been proposed. A core, which will become the center of the ball, is press molded and placed in a mold for the intermediate layer. The mold is provided with holding pins that are designed so as to be able to move forward or backward while being inserted in a plurality of holes that are connected to the cavity of the mold. By holding the core with the holding pins that are projected from the holes, the core and the intermediate layer are concentrically aligned. Then, the material that will form the intermediate layer is poured into the mold and the holding pins are removed from the cavity of the mold just before the material is completely cured. Here, the material for forming the intermediate layer is in the condition just before being completely cured and exhibits quite a high hardness, therefore allowing the position of the core to be retained. Then, by covering the completely cured intermediate layer with a cover, it is possible to obtain a golf ball in which the core and other layers are concentrically aligned with accuracy.
However, the mold and the unit for controlling the holding pins in this method become complicated, increasing the manufacturing cost. Furthermore, in this method, it is difficult to adjust the clearance between the holes of the cavity and the holding pins. If the clearance is too small, it becomes difficult to smoothly move the holding pins in the forward or backward direction; on the other hand, if the clearance is too large, the intermediate layer will flow from the gaps between the holes and holding pins.
The present invention aims to solve the above problems. The first object of the present invention is to provide a multi-piece golf ball having both high ball bounce resilience and a soft feel when hit.
The second object of the present invention is to provide a multi-layered, multi-piece golf ball in which the accurately concentric alignment of the core with respect to the intermediate layer is readily achieved, and a method for manufacturing the same.